Hundreds of millions of internal combustion engines in today's civilization employ poppet valves in the cylinders of those engines to allow intake gases to enter and exhaust gases to exit the engine cylinders. The valves are each translated cyclically between a seated position and a position of maximum lift. The predominant type of valve-lifting action in use is a spring-return action, in which a cam member imparts lift to a valve. The valve is then restrained at its greatest lift by spring forces that return it to its seated position. This is, however, less than ideal for many important reasons.
The magnitude of spring force that must be employed to return a valve to its seat is defined by the maximum kinetic force the valve's intended movement produces, which occurs at the engine's highest rotational velocity. But this spring force, permanently built into the action, must then be opposed by the valve lift mechanism with each and every revolution of the engine. Thus even at lower engine speeds, at which most engines are usually operated, valve-lifting components suffer from kinetics which occur only at high speed. Typical among these components is the cam's driving belt, which is subject to shearing of its teeth even at engine startup. Further, at high engine RPMs the valve kinetic forces can in practice actually exceed the spring restraints and produce so-called “valve float,” in which a valve makes an uncommanded departure from its engagement against the lift imparting surface of its cam. This inhibits engine performance strongly.
Spring-return actions typically flex the spring the entire distance of the valve's motion, which places deep repetitive stress cycling on the spring. Spring failure is known to occur thus, resulting in catastrophic contact between the valve and other engine components. Another problem with spring-return actions is that the spring itself has a mass and is moved vigorously in operation. This makes the spring oscillate internally and this oscillation is known to bounce the valve open once it has been seated, the spring then re-seating it. This produces engine inefficiencies.
A spring-return action must deliver force to accelerate its valve from its seat and simultaneously overcome the spring force on that valve. This double-loading tends to overload the driving cam face and its cam follower, resulting in metallurgical distress to each. Further, the double-loading places heavy cyclic torsional loading on the cam itself, causing a pulsing feedback through its driving components. Cams are known to suffer torsional resonance and breakage under their double-loading.
Industry has adopted cam-lift profiles to drive the valves in their cycles, which profiles are moderate kinetically. While assisting slightly in the issues described above, these moderated profiles compromise cylinder ventilation. Less powerful engines with more pollution and worse fuel efficiency are the result.
A valve action that does not depend on spring force for the return of its poppet valve to a seated position has been applied with some success in industry. This is called a “positive return” action, also known as “desmodromic.” A positive return action relies on mechanical engagement between a driving member, usually a cam, and its driven valve to return the valve to its seated position. Thus the kinetics of valve return are provided for, without resort to a spring.
Yet for all the advantages the positive return action offers, it comes with one very definite design issue. Whatever the mechanism is that drives the return of the valve to its seat, it either 1) exactly stops its drive at the seat, or 2) its drive does not seat the valve, or 3) it overtravels the seated position.
For a positive return valve action to exactly stop driving its valve at the seat is possible for a newly adjusted valve that experiences no significant thermal expansion. Yet a valve is in constant flux of seating-precision due to extremes of thermal influences and to erosion of its valve seat in usage. As a result, expensive and mechanically demanding maintenance of such actions limits their practicality.
For a valve action to not return the valve to its seat means leakage of gases under high pressure and temperature past the valve. This causes inefficiency of the engine and can cause the valve to become overheated and thus get warped and scorched.
When a valve is seated and then urged a distance further by its driving mechanism, “overtravel” of the valve drive is entered. For a rigid driving mechanism, overtravel causes excessive noise and rapid valve seat regression, and can cause fracture of critical components.
To resolve overtravel issues in desmodromic actions, many designs provide yielding at valve seating by incorporating a short-acting spring into the valve linkage. Whereas this can produce acceptable valve seating in overtravel, it also leaves a kinetic problem in place at the valve's greatest lift. Specifically, the suspension of the valve mass on its spring while undergoing extreme kinetic vigor results in deflection of the valve past its intended limit of motion. Heavy spring force must then be used to restrain the valve mass, which in turn causes the valve seating force to be excessive. Oscillations of the valve-mass and spring system can also result that cause the valve to experience re-seating.
Although hydraulic actuators and electromechanical (EM) actuators may be employed as drivers to directly position poppet valves, such positive return actions require an exacting valve landing precision that is expensive and technically difficult to attain. An EM actuator may require significant electrical power to maintain valve seating, due to a powerful spring in the action that accelerates the valve from its seat.
Ballot, in U.S. Pat. No. 1,633,882 teaches a cam follower that contacts a lifting-cam surface with one follower arm and also contacts a returning-cam surface with another follower arm. The follower is thus continuously in contact with both driving lobes of a single cam. To allow for overtravel during valve seating, a resilient member forces a returning-cam contact face against the cam. This places unending contact force against the cam lobes which results in wear and friction.
Parsons, in U.S. Pat. No. 4,898,130 teaches a pivoting guide member with a pin member moved along it. The pin member's movement within the guide member causes the guide member to positively return its linked valve nearly to its seat. The spring that forces the guide member to seat the valve is flexed through the entire valve travel range, placing deep repetitive stress cycling on the spring. The spring force on the guide member is leveraged against the pin member as the valve approaches its greatest lift. This produces excessive friction and wear on the guide and pin members.